The present invention relates to connectors for coaxial cables, and in particular for connectors having a center terminal that connects with the inner conductor of a coaxial cable.
Mechanically and electrically stable connections between the inner conductor and outer conductor of a coaxial cable and the corresponding inner-terminal (or center-terminal) and main body of the connector are known. These stable connections are typically brought about by axial displacements of the various parts of the connector relative to each other such that these displacements are transformed into corresponding inwardly directed radial displacements of contact surfaces of the connector exerting a sufficiently strong pressure against the corresponding inner and outer conductors of the cable. In order to obtain reliable electrical and mechanical connections, these contact surfaces are furthermore often provided with threads or protrusions which may penetrate the surface of the inner conductor, thereby contributing to increased reliability of the connections.
A number of such connectors are known. For example, EP 0 994 527 by the applicant discloses a coaxial connector provided with threads on the contact surface between the inner conductor of the cable and the center terminal of the connector.
U.S. Pat. No. 5,595,502 discloses a connector for a coaxial cable having a hollow inner conductor, where the center terminal of the connector during mounting of the cable on the connector is brought into the hollow inner conductor, and where the portion of the center terminal inserted into the inner conductor is furthermore provided with threads engaging the inner surface of the hollow conductor.
U.S. Pat. No. 6,120,314 discloses a plug connector for the electrically conductive connection of conductor tracks on a board to at least one coaxial cable where the connector is provided with an insertion duct comprising two tubular sections being able to accommodate the inner- and outer conductor of a coaxial cable, respectively. These sections are both provided with inwardly directed protrusions which during mounting of the cable in the connector are brought to penetrate the insulation material around the inner- and outer conductors, respectively, and exert a strong pressure against the surface of the corresponding conductor.
While the arrangement of protrusions of threads as exemplified by the above disclosures leads to a more stable mechanical attachment of the respective conductor of the cable to the corresponding conductor in the connector, such arrangements may nevertheless be undesirable as they may lead to a deterioration of the transmission of high frequency signals at the contact surfaces between the cable and the connector, especially at the interface between the inner conductor of the cable and the corresponding center terminal of the connector, caused, for instance, by the inner conductor of many known coaxial cables being formed with an aluminum core which is provided with a very thin cladding of another conductive material such as copper. At high frequencies the signal current practically takes place only through the very thin cladding due to the skin effect, and local destruction of this cladding can be caused by penetration of the threads or protrusions leading to local impedance discontinuities which tend to degrade signal transmission. It is hence desirable to provide attachment means, especially between the inner conductor of the cable and the center terminal of the conductor, that attains high stability and reliability of connection without introducing the degradation of the electrical signal.
The application of various supporting structures made of a dielectric material in electrical connectors or coaxial connectors is known, for instance to fix the center terminal of the connector appropriately within the main body of the connector, to transmit pressure between various parts of the connector during mounting of the connector on the cable, and to act as a mechanical abutment or backstop for various displaceable parts of the connector. Very often these structures are exposed to large mechanical stress both in use and during mounting of the connector, and these structures should be able to withstand such stress without unacceptable deformations or failure, often over a wide range of temperatures, humidities and even in the presence of chemical agents that may increase the risk of damage to the structures. Within the art it is known to apply, for instance, PEHD or TPX for such structures, but these materials suffer from a number of drawbacks, such as being too soft to provide a consistent attachment of the center terminal to the main body of the connector.
An object of the present invention is to provide a connector for a coaxial cable which provides a firm and reliable electrical and mechanical contact between the inner connector of the cable and the center terminal of the connector while reducing the possibility of degradations of signal propagation at the interface between the inner conductor and the center terminal.
In preferred embodiments, the present invention relates to a connector for a coaxial cable, the cable comprising an inner conductor, the connector comprising: a main body having an outer surface and an inner surface defining a generally cylindrical main body chamber; a tubular support member disposed within the main body chamber and having an inner surface defining a generally cylindrical tubular support member chamber; a center terminal member disposed within the inner tubular member chamber, the center terminal member comprising a sleeve portion having an open end adapted to receive the inner conductor of the cable; an axially movable tubular member disposed within the main body chamber; and a tubular outer bushing having one end adapted to receive the coaxial cable and an opposite end capable of engaging the main body, wherein the tubular outer bushing is capable of axially displacing the axially movable tubular member within the main body chamber. Relative axial movement between the main body and the tubular outer bushing causes the axially movable tubular member to deflect the sleeve portion of the center terminal member radially inwardly, thereby causing the sleeve portion to engage the inner conductor of the terminal. The sleeve portion may further comprise a reduced thickness portion adjacent the protrusion.
In a preferred embodiment, the sleeve portion of the center terminal member has a protrusion extending radially inwardly and having a contact surface for contacting the inner conductor of the cable. Preferably, the protrusion of the sleeve portion has an edge adapted to anchor the protrusion on the surface of the inner conductor of the cable, thereby resisting relative axial movement of the cable with respect to the connector. Preferably, the protrusion has a distal end surface adapted to reduce frictional resistance to the insertion of the inner conductor of the cable into the sleeve portion. Preferably, the sleeve portion of the center terminal member has a beveled surface facing radially outwardly for engaging the axially movable tubular member. Preferably, the axially movable tubular member has a mating beveled surface adapted to engage the beveled surface of the sleeve portion.
In a preferred embodiment, the sleeve portion comprises at least one longitudinal slit.
Preferably, the connector further comprises a ferrule disposed within the tubular outer bushing, wherein the ferrule is adapted to engage the outer conductor of the cable. Preferably, the ferrule is capable of contacting the axially movable tubular member and the tubular outer bushing, wherein relative axial movement between the main body and the tubular outer bushing is capable of causing the ferrule to contact the outer conductor of the cable and the main body.
Preferably, the connector further comprises a tubular inner bushing adapted to surround a portion of the inner conductor of the cable and to reside within a portion of the cable disposed radially outwardly from the inner conductor of the cable. Preferably, the tubular inner bushing comprises an end capable of radially supporting a portion of the ferrule. Preferably, the tubular inner bushing has an end capable of contacting and axially displacing the axially movable tubular member upon relative axial movement between the main body and the outer tubular bushing.
Preferably, at least one of the tubular support member, the axially movable tubular member, and the tubular outer bushing is made from a dielectric material comprising cycloolefincopolymer. Preferably, the cycloolefincopolymer is an amorphous, transparent copolymer based on cyclic and linear olefins according to the formula 
Preferably, the dielectric constant of the dielectric material is less than 3.5. Even more preferably, the dielectric constant of the dielectric material is less than 2.5.
In other preferred embodiments, the present invention relates to a method of forming a coaxial connector to be attached to a coaxial cable, the method comprising: forming a dielectric structure from cycloolefincopolymer, the dielectric structure being adapted to receive the coaxial cable; providing a support structure for supporting the dielectric structure; and assembling the support structure and the dielectric structure to form the coaxial connector. Preferably, the cycloolefincopolymer is an amorphous, transparent copolymer based on cyclic and linear olefins according to the formula 
Preferably, the dielectric constant of the dielectric structure is less than 3.5. Even more preferably, the dielectric constant of the dielectric structure is less than 2.5. In a preferred embodiment, the cable comprises an inner conductor, and the connector comprises: a main body having an outer surface and an inner surface defining a generally cylindrical main body chamber; a tubular support member disposed within the main body chamber and having an inner surface defining a generally cylindrical tubular support member chamber; a center terminal member disposed within the inner tubular member chamber, the center terminal member comprising a sleeve portion having an open end adapted to receive the inner conductor of the cable; an axially movable tubular member disposed within the main body chamber; and a tubular outer bushing having one end adapted to receive the coaxial cable and an opposite end capable of engaging the main body, wherein the tubular outer bushing is capable of axially displacing the axially movable tubular member within the main body chamber; and the dielectric structure is at least one of the tubular support member, the axially movable tubular member, and the tubular outer bushing.
In a preferred embodiment, the present invention relates to a connector for a coaxial cable which includes a center terminal with an end portion for connection to the inner conductor of the cable, the end portion having an annular contact surface longitudinally extending over a predefined distance and protruding radially inwardly from an inner circumferential surface to establish electrical and mechanical contact between the center terminal and the inner conductor of the coaxial cable. The dielectric structures in the connector are advantageously made from a material having a dielectric constant less than 3.5.
Reference will now be made in detail to the present preferred embodiments of the invention, examples of which are illustrated in the accompanying drawings. An exemplary embodiment of a segmented core refractive index profile in accordance with the present invention is shown in each of the figures.